High frequency tuned amplifier circuits



April 1939- B. D. H. TELLEGEN ET AL 2,155,025

HIGH FREQUENCY T UNED AMPLIFIER CIRCUITS Filed May 22, 1935 INVENTORS BERNARDUS D.H.TELLEGEN CAREL VAN LOON BY ATTORNEY Patented Apr. 18, 1939 HIGH FREQUENCY TUNED AMPLIFIER CIRCUITS Bernardus D. H. Tellegen and Carel J. van Loon,

Eindhoven,

Netherlands, assignors to N. V.

, P h i l i p s Gloeilampenfabrieken, Eindhoven,

Netherlands Application May 22,

1935, Serial No. 22,772

In'Germany June 12, 1934 8 Claims.

This invention relates to a circuit arrangement for amplifying high, or intermediate, frequency electrical oscillations, which has the property that the amplification is substantially uniform over a definite frequency band, whereas other frequencies outside this frequency band are suppressed;

In a known amplifying arrangement the succeeding amplifying tubes are coupled together through tuned oscillatory circuits. Consequently both the grid and the anode circuit of an amplifying tube comprise an oscillatory circuit, and these are slightly detuned with respect to one another in order to widen the resonance curve of the amplifier. However, such an arrangement has various defects. Firstly, the resonance curve of the amplifier becomes unsymmetrical in many cases, i. e. theoscillations within the amplified frequency band are not uniformly amplified. O Secondly, only very slight tolerances are admitted in the tunings of the oscillatory circuits, which gives rise to diificulties in the manufacture.

According to our invention better results are achieved with an arrangement in which the oscillatory circuits in the grid and the anode circuit of an amplifying tube are coupled together across an ohmic resistance.

In such an arrangement according to the invention the oscillatory circuits per se may receive the same tuning. However, the combined arrangement behaves like two coupled circuits with two resonance frequencies that may differ from each other.

The invention will be more clearly understood by reference to the accompanying drawing, wherein Fig. 1 illustrates a circuit embodying the invention; Fig. 2 shows a pair of resonant curves obtained under different conditions of operation of the circuit in Fig. 1; and Figs. 3 to 9 inclusive show different circuit modifications according to the invention.

Figure 1 represents an amplification stage of a high, or intermediate, frequency amplifier which comprises an amplifying tube l, for instance a screened grid tube, and two tuned oscillatory circuits L1-,C1 and Lz, C2. The grid of the amplifying tube is connected to the anode across a resistance R which may be variable. If the capacity between the anode and the control grid be negligible, it can be proved that due to the coupling of both oscillatory circuits L1, C1 and L2, C2 effected by the resistance R and the tube I, the combined arrangement behaves like two coupled circuits in such a manner that in the 55 case of a sufiiciently close coupling, i. c. with a hypercritical coupling, two resonance frequencies W1 and W2 (curve A in Fig. 2) occur, so that when both oscillatory circuits are tuned to the same frequency a perfectly symmetrical resonance curve is ensured.

By modifying the resistance R, the coupling, and consequently the shape of the resonance curve, can be modified within wide limits. If, as was assumed in the above example, the tube connected between both oscillatory circuits, is provided with a screen grid, the anode-control grid capacity, in so far as it is determined by the tube, may be rendered negligible. Since, however, the resistance R is connected between these electrodes, the anode-control grid capacity is increased on the one hand by the natural capacity of the resistance itself, and, on the other hand, by the capacity of the connecting lines, and these capacities may attain such a value that the working of the arrangement is affected. If the capacity becomes too high, then both resonance frequencies are unevenly damped due to which the resonance curve of the combined arrangement becomes unsymmetrical, about as is shown in Fig. 2 by the curve B. I

A coupling in which the above influence of the coupling capacity connected in parallel with the coupling resistance is reduced is shown in Fig. 3, where the coupling resistance R is interposed between the tappings of the coils L1 and L2. In this case the resistance R may be materially smaller than in the arrangement represented in Fig. 1, so that the self-capacity C lying in parallel with the resistance affects the coupling to a lower degree.

A similar circuit arrangement is represented in Fig. 4 in which the coupling resistance" R is not connected between tappings of both coils L1 and L2, according to the preceding example, but is connected to the junctions of the condensers C1, C3 and C2, C4. Such an arrangement has the advantage that when the condensers C1 and C2 are variable, as is the case, for instance, in tunable high frequency amplifiers, the coupling of both oscillatory circuits varies with the tuning in such a manner that the width of the frequency band allowed to pass remains substantially constant. However, the width of the band may be adjusted at will for any tuning by modifying the resistance R. Again, the condensers C3 and C4 may also be connected in series with the inductance coils L1 and L2 instead of being connected in series with the condensers C1 and C2.

It is found, however, that both in the arrangement shown in Fig. 3, and. in that shown in Fig. 4, no perfectly symmetrical resonance curve can be obtained, because the coupling between the oscillatory circuits depends not only on the resistance R, but, also, on the value of the inductances between the tappings of the coils L1 and L2 or on the value of the condensers C3 and C4 and on their losses.

A perfectly symmetrical resonance curve can be achieved by means of the arrangement shown in Fig. 5, in which a coil L is connected in series with the coupling resistance R. It can be proved that by this arrangement for a definite tuning of the tuning circuits L1, C1, C3 and L2, C2, C4 both resonance frequencies of the coupled system are damped in the same manner, thus ensuring a symmetrical resonance curve.

However, this holds good only for a particular frequency to which both oscillatory circuits are tuned, so that this coupling may be used with advantage in intermediate frequency amplifiers which, as is well known, have a fixed tuning. For high frequency amplifiers with variable tuning a resonance curve which is symmetrical for every tuning can be ensured by means of the circuit arrangement illustrated in Fig. 6, in which a coil L is connected in parallel with the coupling resistance R. If desired, a further inductance coil L may be connected in series with the parallel connection of the resistance R and the inductance L as shown in Fig. 6a. In this manner it is possible to render the resonance curve symmetrical in a very broad frequency range even when, as is the case for instance in radio receiving sets, the coils L1 and L2 can be partly shortcircuited for receiving two or more ranges of wave lengths.

In all of the above circuit arrangements according to the invention, the coupling resistance was connected in such a manner to both oscillatory circuits that the coupling became closer, and the resonance curve consequently became wider, as the resistance was reduced. It is, also, possible to couple both oscillatory circuits with each other by means of a resistance in such a manner that a sharper resonance curve is obtained. Such an arrangement is represented in Fig. '7, from which it appears that the coupling resistance R is interposed both in the oscillatory circuit L1, C1 and in the oscillatory circuit L2, C2. When calculating the coupling factor for this connection it is found that the square thereof is a negative value which is physically to be conceived so that undamping of the system occurs.

Similar results can be obtained with the arrangements according to Figs. 8 and 9. In the arrangement represented in Fig. 8 the coupling resistance is connected between the grid of the amplifying tube and the junction of a condenser C4 with the coil L2; and in the arrangement shown in Fig. 9 the resistance R is connected on the one hand to the junction of the condenser C3 with the tuning condenser C1, and on the other hand to the junction of the condenser C4 with the coil L2. Both arrangements allow a sharper resonance curve of the system to be achieved.

What is claimed is:

1. A band pass amplifier for radio frequencies comprising an electron discharge tube provided with input and output electrodes, a variably tuned circuit connected to each of said input and output electrodes, and coupling means including an inductance and a shunt resistance connected between said variably tuned input and output circuits, said coupling means providing suiliciently close coupling between said tuned circuits whereby a predetermined band of frequencies will be amplified substantially uniformly, said coupling means varying with the tuning in such manner that the width of the frequency band pass remains substantially constant.

2. An amplifier stage comprising an electron discharge device provided at least with a cathode, control grid and anode, a tuned input circuit connected between cathode and control grid, a tuned output circuit connected between cathode and anode, and an ohmic resistance connected between high potential points on said two tuned circuits.

3. An amplifier stage comprising an electron discharge device provided at least with a cathode, control grid and anode, a tunable input circuit, comprising an inductance and a shunt variable capacity, connected between cathode and control grid, a tunable output circuit, comprising an inductance and a shunt variable capacity, connected between cathode and anode, and an ohmic resistance connected between high potential points on said two tunable circuits.

4. The invention according to claim 3 wherein the ohmic resistance is connected between high potential points on the respective inductances of said tunable circuits.

5. The invention according to claim 3 wherein the ohmic resistance is connected between high potential, intermediate points on the respective inductances of said tunable circuits.

6. The invention according to claim 3 wherein a capacity is connected in series with each variable capacity, said series-connected capacities shunting their respective inductances, and the ohmic resistance is connected between the common terminals of said series-connected capacities.

7. The invention according to claim 3'wherein a capacity is connected in series with each variable capacity, said series-connected capacities shunting their respective inductances, and wherein an inductance is serially connected to the ohmic resistance and together are connected between the common terminals of said series-con nected capacities. V

8. The invention according to claim 3 wherein a capacity is connected in series with each variable capacity, said series-connected capacities shunting their respective inductances, and wherein an inductance is connected in shunt to the ohmic resistance and together are connected between the common terminals of said series-connected capacities.

BERNARDUS D. H. 'I'ELLEGEN. CAREL J. v. LOON. 

